3GPP standards and in particular 3GPP 5.08 requires a mobile device to select a cell to connect to based only on signal strength. As a result, all mobile devices in practice tend to connect to the nearest and strongest cell, sharing its packet data channels, and potentially causing data congestion. This is true even when another cell that is only slightly weaker is available and sits idle. This is true of other types of wireless networks as well.
While in circuit switched channels a handover mechanism exists, for packet data all mobile devices multiplex available channels, thereby resulting in low throughput rates when a large number of mobile devices are connected. The length of data calls also tends to be shorter than voice (circuit switched) calls, making handover during a data call impractical. A data call is referred to herein as a temporary block flow (TBF).
For circuit switched channels, the network initiates the handover. However, network controlled cell reselection is not widely deployed for data. There are existing mechanisms, such as network controlled cell selection, in place that allow for mobiles to be directed to specific cells by the network. These mechanisms typically involve some per-mobile overhead in communicating measurements to the network. Such a mechanism is useful for dealing with a relatively small number of mobiles (less than 10) doing relatively long data transfers. However, for cases where there are hundreds of mobiles doing very short and sporadic transfers, the overhead of network controlled cell selection will cause sufficient overhead to reduce the overall throughput of the network.
For congested cells it is undesirable for a network to control cell reselection since every mobile device connected to the cell would need to constantly send measurement reports to the cell. This would take up packet resources and further reduce network performance.